Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-03-20
2002-02-26
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06350019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to ink jet heads and ink jet printers, and more particularly to an ink jet head which uses a thin film piezoelectric element as a means of jetting ink, and to an ink jet printer which uses such an ink jet head.
Most inexpensive color printers are ink jet printers which use the ink jet head, because ink jet head does not generate noise and the ink jet printer can realize color printing at a low cost compared to electrophotography printers.
Recently, there are demands to realize a high resolution by the ink jet printer, and rapid developments are being made to minimize the drop diameter of the ink which is jetted from the ink jet head. Moreover, there are demands to realize an ink jet printer having a structure suited for mass production, while satisfying the demands to realize a high performance.
2. Description of the Related Art
FIG. 1
is a diagram showing an example of an ink jet printer. An ink jet printer
10
shown in
FIG. 1
includes an ink jet head
11
which is mounted on a lower surface of a carriage
12
. This ink jet head
11
is positioned between a feed roller
13
and an eject roller
14
, and confronts a platen
15
. The carriage
12
has an ink tank
16
, and is movable in a direction perpendicular to a drawing paper on which
FIG. 1
is drawn.
A paper
17
is pinched between a pinch roller
18
and the feed roller
13
, and is transported in a direction A in a state pinched between a pinch roller
19
and the eject roller
14
. The ink jet head
11
prints on the paper
17
when the ink jet head
11
operates and the carriage
12
moves in the direction perpendicular to the drawing paper. After the printing, the paper
17
is accommodated within a stacker
20
.
FIG. 2
is a perspective view showing an important part of an ink jet head. As shown in
FIG. 2
, an ink jet head
30
, which corresponds to the ink jet head
11
described above, includes a nozzle plate
33
formed with nozzles
32
from which ink is jetted, pressure chambers
35
and ink passages
40
which are respectively formed in correspondence with each of the nozzles
32
, a driving part
31
forming one wall of each of the pressure chambers
35
, a common ink passage
39
for supplying the ink to each of the pressure chambers
35
, and a main body
36
. The pressure chambers
35
and the common ink passage
39
are integrally formed in the main body
36
.
The driving part
31
includes piezoelectric elements
37
which are provided with respect to each of the pressure chambers
35
on a vibration plate
34
which forms one wall of each of the pressure chambers
35
in common. The vibration plate
34
also forms the common electrode of each piezoelectric element
37
. Individual electrodes
38
are provided on the top surface of the corresponding piezoelectric elements
37
. The driving part
31
forms a bimorph structure by the piezoelectric elements
37
and the vibration plate
34
. When a driving signal from a controller is applied on the individual electrode
38
, the corresponding piezoelectric element
37
is distorted so as to contract in an in-plane direction of the vibration plate
34
. Hence, the driving part
31
deforms towards the corresponding pressure chamber
35
as indicated by a phantom line in
FIG. 2
, and the ink drop is jetted from the corresponding nozzle
32
. When the driving signal is no longer applied to the individual electrode
38
, the driving part
31
is restored to the flat non-deformed state, thereby supplying the ink from the common ink passage
39
to the corresponding ink chamber
35
.
According to such a bimorph structure, it is possible to obtain a large volume displacement with respect to a small distortion of the piezoelectric element without requiring complex structure for fixing piezoelectric element end. For this reason, this bimorph structure is suited for mass production.
When forming the bimorph structure, a plate-shaped piezoelectric element base is cut into a plurality of narrow piezoelectric elements, and the piezoelectric elements are fixed on the vibration plate by means of an adhesive agent or the like. But because of this structure, the piezoelectric element may come off from the vibration plate when the piezoelectric element is greatly deformed, and there is a problem in that it is difficult to form an ink jet head having a satisfactory printing efficiency. Furthermore, since it is necessary to carry out a process of fixing the mechanically cut piezoelectric elements on the vibration plate, it is difficult to miniaturize the pressure chambers and the piezoelectric elements.
On the other hand, a relatively small bimorph structure can be made by forming the piezoelectric elements by use of the printing technique. In other words, a common electrode is formed on a vibration plate which is made of a highly heat resistant material such as ceramics, and a paste of the material which forms the piezoelectric elements is formed and patterned on the vibration plate by use of the screen printing technique, and then baked. It is difficult to increase the density of the piezoelectric elements having this structure because the piezoelectric elements are formed by use of the printing technique, and in addition, the piezoelectric elements formed are brittle mechanically and electrically. As a result, there is a problem in that the piezoelectric element breaks when the piezoelectric element is greatly deformed. Consequently, it is necessary to increase the area of the pressure chambers and the piezoelectric elements as much as possible, in order to compensate for the small tolerable deformation of the piezoelectric elements.
However, according to the method which forms the bimorph structure using the printing technique, it is difficult to mass produce the piezoelectric elements having a thickness of less than 15 &mgr;m. For this reason, the pressure chamber must have a width of at least 200 &mgr;m in order to mass produce the ink jet heads having a satisfactory printing efficiency, and the pitch of the nozzles cannot be made small.
On the other hand, in order to make the piezoelectric element thin, it is possible to use the thin film technique such as sputtering, instead of using the screen printing technique. But when the thin film technique is used, the thickness of the piezoelectric element on the order of several tens of &mgr;m is too thick for mass production, in that it takes too much time to form such a thick layer by the thin film technique. Hence, if an attempt is made to use a piezoelectric element having a thickness on the order of only several &mgr;m which can be formed by the thin film technique, it is impossible to generate a sufficiently large pressure with respect to the ink due to the bimorph structure if the size of the pressure chamber is the same as that for the thick piezoelectric element having the thickness of less than 15 &mgr;m. As a result, it is impossible to obtain a sufficiently large volume displacement by use of such a thin piezoelectric element.
Furthermore, if the size of the pressure chamber with respect to the thin piezoelectric element is reduced to the same proportion as the size of the pressure chamber with respect to the thick piezoelectric element, the pressure chamber becomes too small. Consequently, the ink drop which can be jetted by the combination of the thin piezoelectric element and the small pressure chamber becomes extremely small, that is, only on the order of a fraction of 1 pl or less, where 1 pl=10
−12
1=10
−15
m
3
.
It is possible to arrange the nozzles at a small pitch when the pressure chamber is small, and a high-quality image can be printed when the ink drop is small, thereby making it possible to realize a high-performance ink jet head. However, the ink drop must be at least 2 pl in order for a corresponding printed dot to be recognizable by the human eyes, and for this reason, the image quality cannot be improved even if the printing is carried out using ink drops smaller than
Koike Shuji
Oikawa Koichi
Sakamoto Yoshiaki
Shingai Tomohisa
Armstrong Westerman Hattori McLeland & Naughton LLP
Barlow John
Brooke Michael S
LandOfFree
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